 Kepler-186 is indicated by the red circle(2MASS J band image) | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Cygnus[2] |
| Right ascension | 19h 54m 36.6535s[3] |
| Declination | +43° 57′ 18.026″[3] |
| Apparent magnitude (V) | 15.29[4] |
| Characteristics | |
| Evolutionary stage | Main sequence[3] |
| Spectral type | M1V[5] |
| Astrometry | |
| Proper motion (μ) | RA: 2.171(18)mas/yr[3] Dec.: −4.363(20)mas/yr[3] |
| Parallax (π) | 5.6336±0.0169 mas[3] |
| Distance | 579 ± 2 ly (177.5 ± 0.5 pc) |
| Details | |
| Mass | 0.544 ± 0.02[5] M☉ |
| Radius | 0.523 ± 0.02[5] R☉ |
| Luminosity (bolometric) | 0.055+0.011 −0.006[5] L☉ |
| Temperature | 3755 ± 90[5] K |
| Metallicity[Fe/H] | −0.26 ± 0.12[5] dex |
| Rotation | 34.404±0.075 days[6] |
| Age | 4.0 ± 0.6[5] Gyr |
| Other designations | |
| KIC 8120608[7],KOI-571, 2MASS J19543665+4357180, Gaia DR2 2079000330051813504 | |
| Database references | |
| SIMBAD | data |
Kepler-186 is amain-sequenceM1-typedwarf star, located 177.5parsecs (579 light years) away in the constellation ofCygnus. The star is slightly cooler than thesun, with roughly half itsmetallicity. It is known to have five planets, including the first Earth-sized world discovered in thehabitable zone:Kepler-186f.[8] The star hosts four other planets discovered so far, though they all orbit interior to the habitable zone.
Within two first years of gathered data, the signals of four inner planetary candidates were found. Discussion of planets in the system was taking place in August and November 2013.[9] In February 2014, those planets were confirmed through the "verification by multiplicity" method. The fifth outermost candidate was confirmed in the same manner in April 2014.[10] The possibility that the signals in the light curve of the star were actually from something else has been ruled out by an investigation with theW. M. Keck andGemini Observatories, usingspeckle imaging andadaptive optics techniques, which, while unable to resolve the planets, were able to rule out other possibilities than the system of planets.[11]
As theKepler space telescope observational campaign progressed initial identifications of systems were entered in theKepler Input Catalog (KIC), and then progressed as a candidate host of planets asKepler Object of Interest (KOI). Thus Kepler-186 started asKIC 8120608 and then was identified asKOI 571.[12] Planetary candidates were detected around the star byNASA'sKepler Mission, a mission tasked with discovering planets intransit around their stars. The transit method that Kepler uses involves detecting dips in brightness in stars. These dips in brightness can be interpreted as planets whose orbits pass in front of their stars from the perspective ofEarth, although other phenomena can also be responsible which is why the term planetary candidate is used.[13]
Outside of the Kepler project, the2MASS survey catalogued this star as2MASS J19543665+4357180.[1]
A number of previously unknown measurements of the star are known.[14][15] In the infrared/microwave EM spectrum[1] itsH band magnitude is 11.605,J band magnitude is 12.473, and itsK band magnitude is 11.605. In the visualPhotometric system magnitude it is 14.90(R)(towards the red end of the visual spectrum) and 16.40(B)(the blue end of the spectrum)[14] (see alsoApparent magnitude.) It is aBY Draconis variable changing brightness slightly, probably fromstar-spots, with a period of 33.695 days.[16]
The star is an M-typered dwarf, bordering on being a K-typeorange dwarf, with a mass 0.544 times that of theSun's and a density of5.29 g/cm3.[5]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | ~1.24 M🜨 | 0.0343±0.0046 | 3.8867907 | <0.24 | 83.56° | 1.07±0.12 R🜨 |
| c | ~2.1 M🜨 | 0.0451±0.0070 | 7.267302 | <0.24 | 85.94° | 1.25±0.14 R🜨 |
| d | ~2.54 M🜨 | 0.0781±0.0010 | 13.342996 | <0.25 | 87.09° | 1.4±0.16 R🜨 |
| e | ~2.15 M🜨 | 0.11±0.015 | 22.407704 | <0.24 | 88.24° | 1.27±0.15 R🜨 |
| f | 1.44+2.33 −1.12 M🜨 | 0.432±0.01 | 129.9444 | <0.04 | 89.9° | 1.17±0.08 R🜨 |
The five planets discovered around Kepler-186 are all expected to have a solid surface. The smallest one, Kepler-186b, is only 8% larger than Earth, while the largest one, Kepler-186d, is almost 40% larger.
The four innermost planets are probablytidally locked, but Kepler-186f is farther out, where the star'stidal effects are much weaker, so there may not have been enough time for its spin to slow down that much. Because of the very slow evolution of red dwarf stars, the age of the Kepler-186 system is poorly constrained, although it is likely to be greater than a few billion years.[17] There is a roughly 50-50 chance it is tidally locked. Since it is closer to its star than Earth is to the Sun, it will probably rotate much more slowly than Earth; its day could be weeks or months long (seeTidal effects on rotation rate, axial tilt and orbit).[18]
Planetary formation simulations have also shown that there could be one additional non-transiting low-mass planet between Kepler-186e and Kepler-186f. If this planet exists, it is likely not much more massive than Earth. If it were, its gravitational influence would likely prevent Kepler-186f from transiting.[17] Conjectures involving theTitius–Bode law, (and the relatedDermott's law) indicate that there could be several remaining planets to be found in the system - two small ones betweene andf and another larger one outside off.[19] That hypothetical outer planet must have an orbital radius beyond 16.4AU for planetary system to remain stable.[20]
The lowmetallicity of the star at a metallicity (dex) of -0.26, or to put it another way, about half that of the Sun's,[8] is associated with a decreased chance of planets overall and giant planets specifically but an increased chance of Earth sized planets, in a general study of stars.[21]
